In a mobile communication apparatus such as a cellular phone, PDA and the like, a CD player, an MD player, a small tape recorder, a remote control switch, or a small electric electronic apparatus mounted on a motor vehicle, illumination of a switch portion (such as a key top portion) of a key switch is performed. As a light source for such an illuminated switch in which a key top portion of a key switch is illuminated, generally a light bulb or an LED is adopted.
For the illuminated switch a structure having a key top, a switch mechanism portion such as a metal dome switch, a substrate and an LED as a light source is generally used. Incidentally, for mobile communication apparatuses such as a cellular phone and PDA, there is a strong demand to make a key switch thinner, and therefore a LED cannot be arranged under a key top. Accordingly, a common structure is such that an LED is arranged at a separate position from a key top and a switch mechanism portion, and light from the LED is diffused to illuminate the key top portion indirectly from the periphery. However, since the conventional illumination structure is not illumination from immediately under the key top, there are problems such that it is difficult to illuminate the key top portion evenly with adequate brightness, and that the structure becomes thick.
in this aspect, it is proposed to use an EL sheet having an electroluminescence (EL) element for the light source of an illuminated switch (for example, refer to patent documents 1 and 2). The EL sheet is a plane-like light source and has characteristics such that it is excellent for saving space due to its light weight, thinness and high flexibility in shape, and further it has low power consumption. Thus, the EL sheet can be arranged directly between the key top and the metal dome switch. According to an illuminated switch using such an EL sheet, it becomes possible to illuminate a key top from immediately under it.
As described above, the EL sheet is considered to be effective as an illumination light source for a key switch. However, according to results of experiments and considerations conducted by the present inventors, it is revealed that conventional EL sheets have weak points such as failure to illuminate after a short period of time due to keystroke stress from a key top, and malfunction of a switch and impairment of clicking feeling (feeling of pressing a switch) due to the stiffness of the EL sheet.
In a conventional EL sheet, generally, a film made by depositing or applying ITO (indium tin oxide) on a polyester film with a thickness of 75 μm or larger is used as a transparent electrode film. A deposited film of ITO has high light transmittance and high conductivity, but also has defects such as breaking easily by extension/contraction or increasing its electric surface resistance due to mechanical stress or heats. Accordingly, it is revealed that when the EL sheet is bent due to keystroke stress from a key top, a crack is generated in an ITO electrode, which easily causes increase in resistance value, wire breakage and failure to illuminate. According to experiments conducted by the present inventor and others, by increasing the thickness of a base film of the ITO film, it is possible to suppress for a certain degree the failure of the EL sheet to illuminate, but in this case reliability and clicking feeling of the key switch are impaired.
Also, the present inventors have considered of making a transparent electrode using transparent conductive paint made by dispersing transparent conductive powder such as ITO in insulative resin. When a transparent electrode layer is formed using ITO paint or the like, it is possible to suppress for a certain degree the failure of the EL sheet to illuminate, but due to the need of increasing thickness or burning in order to make the resistance value of the transparent electrode to be low resistance, an applied film thereof stiffens during drying and curling becomes large. Therefore, it is difficult to produce the EL sheet using a thin base material. Even when using a thin base material, the transparent electrode layer stiffens due to inclusion of inorganic particles such as ITO. It is found that all of these can be a cause of impairing the clicking feeling. Further, a problem also arises such that a black dot is often generated on the EL sheet during illumination in a high-humidity environment.
By the way, in the aforementioned patent document 1, it is described that a slit is formed at a position along an outer periphery of a metal dome switch of an EL sheet, thereby improving a clicking characteristic. Also, in the patent document 2, an illuminated switch is described in which a transparent electrode film which is made by forming a transparent electrode layer on a base film, is formed in a dome shape, and an EL light emitting portion is formed inside a switch operating portion of this dome shape. Both of them use a deposited film of ITO for the transparent electrode layer, and thus problems such as wire breakage and increase in surface resistance due to the deposited film of ITO are not solved.
[Patent Document 1] JP-A 2002-56737 (KOKAI)
[Patent document 2] JP-A 2004-39280 (KOKAI)